Downsizing, ease of mounting, and reduced requirements for mounting space are demanded of microphones to be mounted in such small equipment as cell phones.
Electret condenser microphones (ECM) are commonly used as such microphones that are provided with good characteristics, easy to handle because of their simple structure, and easy to downsize. They are mounted on a target mounting-board (board of a target equipment) using a surface-mount technique.
FIGS. 11A and 11B show a perspective view and sectional view of a configuration described in Japanese Patent Application Laid-Open No. 2005-192180 (hereinafter referred to as patent literature 1) as an example of a mounting structure for use to mount a microphone on a mounting board. In the figures, reference numeral 10 denotes a microphone and 20 denotes a mounting board (board of a target equipment). Incidentally, internal structure of the microphone 10 is not shown in FIG. 11B.
In this example, as shown in FIG. 11B, no sound receiving sound hole is formed in a capsule 11 which provides a shell of the microphone 10, but a sound hole 13 is formed in a circuit board 12 which covers an opening 11b of the capsule 11. External terminals 14 and 15 are formed on an external surface of the circuit board 12, and soldered to a terminal pattern (not shown) formed on the mounting board 20, thereby mounting the microphone 10 to the mounting board 20. Reference numeral 16 in FIG. 11B denotes a solder layer.
A sound receiving through-hole (sound hole) 21 is formed in the mounting board 20, and the microphone 10 is mounted with the sound hole 13 in the circuit board 12 aligned with the through-hole 21 in the mounting board 20.
With the mounting structure in this example, sound waves from a sound source are captured into inner space of the microphone 10 through the through-hole 21 in the mounting board 20 and the sound hole 13 in the circuit board 12. Thus, in this example, the mounting board 20 can be placed in the equipment in such a way that a parts-mounting surface 20a will face inward in the equipment.
That is, if a sound hole is formed in a front plate 11a on the opposite side of the capsule 11 from the circuit board 12 rather than in the circuit board 12, it is necessary to place the mounting board 20 inside the equipment so that the parts-mounting surface 20a will face the sound source (turn outward) because the sound hole must face the sound source to capture the sound waves from the sound source. In that case, it is necessary to provide a space large enough to accommodate height of the microphone between the mounting board 20 and a housing of the equipment. However, the configuration shown in FIGS. 11A and 11B eliminates the need for such a space, and thus requires less space in order to mount the microphone.
However, with the conventional mounting structure shown in FIGS. 11A and 11B, the sound hole 13 in the circuit board 12 of the microphone 10 and the through-hole 21 in the mounting board 20 are superimposed one upon the other, i.e., the sound hole 13 is placed at a location onto which the through-hole 21 is projected in a height direction of the microphone 10, making it easy for foreign matter such as dust and water droplets to get into the inner space of the microphone 10.
Also, since the positions of the sound hole 13 and through-hole 21 coincide, any foreign matter entering the through-hole 21, for example, during an assembly process or the like may damage a diaphragm in the microphone.
Furthermore, the sound waves, which enter the diaphragm directly through the through-hole 21 and the sound hole 13, are liable to cause pop noise.